I am currently in Norway. My Mac mini is in my apartment in Australia. I can access screen sharing via Back To My Mac. It connects fine, but the screen has only linen - no menu bar, no dock, no anything else. I am fairly sure that is what is actually on the screen and that it isn't a problem with the connection or anything like that (it's happened once before while I was sitting in the same room...)

I know the system is responsive because the one other thing that can poke through my router is the Transmission web interface, and that works just fine. (Unfortunately I seem to have neglected to forward a port for ssh...)

Is there any way I can reboot or otherwise regain full use of the system via the limited screen sharing access that I have?

(Make a note of the number on the last line, in this case 123456789 (this is just an example).

The hostname you need to use for ssh is <computer's local hostname>.<number above>.members.btmm.icloud.com. Replace spaces with hyphens, and omit apostrophes (the default name is "Name's <type of Mac>")

The command I used therefore was:

ssh -2 -6 Jons-mac-mini.123456789.members.btmm.icloud.com

To complete the solution to my problem (rebooting), once logged in via SSH, I used the following command to restart: